Don't be fooled

 Identifying quartz crystals

 

Our last article had a bit of a geopolitical angle – looking at developments in                        Ontario around critical minerals. Today the angles will be more around quartz                         crystal identification. We have quartz that is found on our Bear Lake II claim,                                set in with feldspar crystals in pegmatite intrusions. Our North claim, the most                          recent acqusition is entirely different from the vein dyke fissures on our more                          southerly Bear Lake claim. Here quartz seems to dominate along with pink                           feldspars and a paucity of the typical gneiss country rock to the south.                                 Upturned plates in the forest hint at some catastrophic upheaval that ripped                                 the underlying pegmatties apart - maybe blasting. On lidar we see old tracks                               that lie hidden to casual observation. Leaves camoflague the tracks from some                              70 years back but we suspect that blasting was conducted in the local                                pegmatites as old prospectors look for radioactives. 

 

Josh, our claim neighbor dropped by to show us beautiful glassy quartz that his father had found on our claim before we'd staked it. I was impressed with their clarity and terminations. Like all crystals, the angles between quartz faces are quite distinctive and as you will soon come to realize, there are distinctive faces that are quite typical of quartz, but quartz can vary quite significantly, it can be squat or slender, left or right handed and through these crystals there is a moody darkened cloud that spreads from the edges of some crystals - smoky quartz due to radiation. Usually with enough examination you'll find a tiny pip of uranite and sometimes a rusty halo around the pip, a tension fracture much as you often see in peridot.

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Today we’ll look at four different  approaches to identification that anyone can do with equipment of the most basic variety, the fracture of quartz, its specific gravity, its hardness and its form. Of the many different varieties of quartz, we are simply going to focus on the rock crystal variety as it encompasses much of commonality between what you are likely to see, and in learning about the simple crystal, there is much that can then be extrapolated to the other varieties, like amethyst, citrine and chalcedony.  Habit and color may change, but you will still have a knowledge of the underlying mineral.

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Simple as it may seem, there are so many varieties habits and distorted crystal forms with quartz, but all have the chemical formula of SiO2, but in crystal lattice the quartz is composed of a tetrahedron, a pyramid shape with a silicon atom at the center of the pyramid and four oxygen atoms at each corner of the pyramid, each oxygen atom being shared by 2 other tetrahedra, thus forming a 3D lattice with no particular planes of cleavage; it’s the same cohesive strength  in every direction and so when quartz breaks it fractures like glass, with a concoidal scoop, its insides wavy and much as you would see in chipped glass.

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In the case of glass, it also has no particular direction that is more strongly bonded than any other direction, its atoms are entirely random so you would refer to it being amorphous (without internal structure) – so for totally opposite reasons (no structure thus equal bonding strength in every direction), it also breaks with concoidal fractures.

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Quartz is a common mineral and it comprises about 20% of the earth’s crust. Unlike gold it cannot be separated from the common river pebbles by weight as most of them are very similar to casual comparison. You’ll not find quartz accumulating in a deep crack in the river bed or where the water slows down as minerals that are relatively heavier do. In a flowing stream quartz just acts like every other pebble around it. Now if you are looking at a hand full of pebbles you might have the urge to separate out your quartz crystal from other look-alike’s and to do this you would need a balance scale much as would be used in a high school chemistry class, I found mine at value village. In using this scale you will be comparing the density of the substance that you are weighing to that of water. Simply put, you weigh your crystal as you usually would and then weigh it again in distilled water  at 4 degrees Celsius. That temperat8ure was chosen as a standard as it is at that point that water is most dense.

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When weighing your crystal in water it will displace a certain amount of liquid and it is the weight of that volume of liquid that you are intending to measure your crystal against. It is a basic scientific principle that has been attributed to the observations of the ancient Greek physicist known as Archimedes.

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Plugging your two measurements into the                                                                         required formula you obtain the specific                                                                             gravity of whatever you are weighing – its                                                                         weight relative to the volume of water that it                                                                            has displaced. Quartz is relatively light with                                                                            an SG of 2.65, whereas sapphire comes in                                                                          much heavier at 3.98 – 4.06. This                                                                                     measurement can certainly distinguish between                                                                   quartz and almost any other mineral, however,                                                                         the smaller the specimen being measured, the                                                                         less accurate your weighing becomes.

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Hardness and refractive index on a standard                                                                    polished surface will combine to present the                                                                          luster of a stone. Quartz has a hardness of 7                                                                          and a luster that is said to be vitreous.                                                                                Hardness is a relative measure in its most                                                                                 basic sense when we are talking about gemology. With a hardness of 7 quartz can scratch softer substances like fluorite, calcite or glass, but it cannot scratch topaz. Admittedly most people don’t have a topaz lying around, but the fact it can scratch glass (hardness 5-5.5) at least eliminates the softer minerals from the possibilities. Luster is best judged by looking at the reflected light on the surface of a face or facet. Again it’s a comparative measure. Looking at the luster of amber its said to be resinous – quite dull in fact, and at the other end of the spectrum is a diamond’s luster which is quite brilliant due to its hardness (10) and refractive index which is also high. A diamond is said to have a luster that is adamantine. Keep in mind, without a refractometer, the sum total of these results need to be considered when determining the possible composition of the crystal in front of you. A vitreous luster is something similar to what you would see in the reflection of light from a flat glass surface.

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What now remains is a determination that is to be made by the most valuable instrument that you have got – your eyes!.What does the crystal look like? What is the crystal system and how are the crystal faces arranged?

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So two words to understand before we proceed on, habit and form. Habit is how the crystal appears in a general sense, it has nothing to do with geometry of the individual crystal and everything to do with a relative comparison to the overall appearance in relation to the perfect crystal. So by habit you might describe the crystal as being blocky, botroidal or needle like. Quite clearly the crystal in being called needle like deviates from the usual expectation of a quartz crystal, while when we talk of form we are speaking of the symmetry of the crystal and the angles that the faces meet at. And in the case of quartz there are easily identifiable faces that occur in predictable proximity to each other.

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All crystals fall into one of seven systems and each system is defined by certain symmetries and combinations of forms. A form is either open or closed and the determination is based around whether the form could hold water if poured into it. A closed form would be able to hold water an open form cannot hold water. There are 30 closed forms and 18 open forms. So in the first of the crystal systems, the isometric system, you have only closed forms such as the cube, the octahedron , the dodecahedron etc. and within that system you would get diamond, garnet and fluorite to name a few. All of those crystals can come in a cube shape, or as an octahedron or any of several other shapes.

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In the case of quartz it is found in 2 specific crystal systems, the trigonal system in which alpha – quartz is found and the hexagonal system in which beta quartz is found. Alpha quartz forms at temperatures under 573 degrees Celsius and has a 3 fold symmetry, meaning the arrangement of faces repeats itself 3 times as the crystal is rotated around the c axis – the axis running up its central prism. You would also describe the alpha quartz as being comprised of a 3 sided prism (an open form) and terminated by another open form such as a 3 sided pyramid, a flatted pinacoid or possibly another form that typically terminates a prism. Beta quartz has formed between 573 degrees and 870 degrees. It shows itself as this variety by its 6 fold symmetry. So as you rotate the upright crystal in front of you the same image of termination and prism face repeats itself 6 times to your view.

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So it is not so much in the prism that you will see the distinctive quartz shapes, but rather in the terminations. The prism could be terminated at both ends which is quite unusual and indicative of a crystal that grew in a cavity with plenty of space to spare. In the case of Herkimer diamonds it is the norm as opposed to the exception, however there is one aspect of the quartz prism that is quite distinctive and that is the horizontal striations across the main prism faces. It is important to understand that depending upon the growing conditions face sizes will vary and this explains the many shaped varieties of quartz.

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In the basic quartz there are 3 crystal faces, the ‘m’ face which repeats itself around the prism, the ‘r’ face which is 7 sided and typically the biggest of the termination faces and the ‘z’ face which is a small triangular face between the r faces. Look at your crystal from above its termination, it is probably the easiest way to determine symmetry. Looking from the side, it’s not a question of whether you see the prism or m face repeating itself 6 times, it is the combination of all 3 faces repeating themselves. So in the alpha crystal you will see 6 M faces, but in combination with the Z and R faces you see the same image only 3 times.

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Now comes the really funky part of the quartz crystal, its basic tetrahedrons tend to spiral as they bond in their 3D lattice. It’s called a helix and the spiral can go either to the left or right and the direction is sometimes represented by small x and S faces set along the edge of the R face. Depending upon the placement of the X and S faces  - to the right or left of the top of the prism face you would say that you have a right or left handed crystal. The split is about 50/50 and I am unable to determine why the helix would spiral one way or the other.

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Twin crystals can hamper simple visual identification, but in short what the call the Brazil twin is the most common and it is represented by the X face being on both the left and right of the prism. It represents where both right and left handed crystals have inter-grown.

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So confusing as it might seem, quartz though being common is far from simple. There are many different twinning possibilities aside from the Brazil Twin and as much as you can develop a certain level of confidence in a crystal’s identity, you cannot be certain unless in the case of quartz it is faceted and you employ a refractometer which is beyond the scope of this article.